Conservation and efficiency are important aspects of modem large scale energy distribution systems. Likewise, effective control of load on the power grid is essential to prevent brownouts or spikes in energy costs. When a local utility is unable to meet its local demand, the utility seeks to buy excess capacity from other utilities. Because most of the U.S. and some parts of the rest of North America are connected to the same energy grid, it is not uncommon for utilities in metropolitan areas to buy electricity from utilities in more rural areas. On exceptionally hot summer days, the cost per megawatt-hour of electricity can soar as utilities compete with each other to meet the surge in demand as a result of air conditioners.
When faced with high load demand on the grid, electricity suppliers often offer economic incentives to larger energy consumers, such as factories, manufacturing facilities, and large corporate complexes, to reduce their consumption. The reduction of normal power loads is often referred to as power shedding or load shedding. As customers shed their loads, the demand on the grid reduces, which in turn helps control the demand for and volatility of the costs of electricity.
As part of the load shedding arrangements for energy consumers, different systems for controlling and/or monitoring energy consumption have been developed. For example, in U.S. Patent Application Publication No. 2005/0143865, Gardner teaches a system and method that accepts commands from the local utility company causing controlled devices, such as air conditioner compressors, to reduce consumption for settable periods of time, such as the peak demand period on hot summer afternoons.
Typically, utility companies negotiate predefined load shedding arrangements with large commercial or industrial consumers who have relatively predictable consumption patterns. A large factory or office complex is more likely to have a generally predictable power consumption pattern on a day to day basis, and therefore the potential energy savings are also more predictable. Larger energy consumers tend to have larger equipment that can justify more expensive and elaborate monitoring and control systems.
Unfortunately, the load shedding arrangements developed for these larger energy consumers are not generally applicable when it comes to estimating a predictable power consumption pattern for small commercial or residential consumers. Small commercial and residential energy consumers are defined herein as energy consumers having spaces that are cooled by one or more units with capacities of less than 20 tons. Unlike larger energy consumers, smaller consumers have less predictable energy demands. In addition, as a whole, smaller energy consumers utilize a great diversity of types of equipment having different efficiencies and operating modes, as well as being oversized to the cooling space by varying degrees. Furthermore, the sheer number of small energy consumers presents a substantial challenge in administering a load shedding program, including equipping each piece of equipment for load control at reasonable cost, and issuing credits or other incentive to the energy consumers for participation in the load shedding program in a fair, or uniform, manner.
Several attempts to provide load shedding capabilities to small consumers have been proposed, including the Gardner system mentioned above. In U.S. Pat. No. 6,891,478, Gardner also teaches a system of control devices that individually monitor and control energy consumption of individual appliances, including the air conditioning system, wherein the control is dictated in part based on local utility load shedding requirements. In U.S. Patent Application Publication No. 2006/0036349, Kates teaches a system whereby the local utility may send commands to cooling appliances, such as air conditioners, refrigerators, and freezers; the system, having been calibrated to the individual appliance's optimal performance characteristics, causes the cooling appliances to efficiently reduce power consumption to help meet load shedding requirements. In U.S. Pat. No. 5,675,503, Moe teaches a system that uses historical performance data for a controlled device, such as an air conditioner, to determine how best to cycle the device to meet the load shedding demands communicated by the a local utility while maintaining an indoor temperature within an acceptable range.
Typically, a load shedding arrangement for small energy consumers facilitates remotely controlling the operation of air conditioning units, which are generally the largest single energy consuming appliances within a home or small business. These cooling systems generally include the main components of an evaporator, a condenser, and a compressor. The compressor receives low pressure gaseous working fluid from the evaporator and delivers it as a gas to the condenser. The standard method of cycling cooling systems (air conditioners) for utility demand relief is to switch them off for some percentage of the time. Typically, they are cycled off 50% or 15 minutes out of every half hour. If the cooling system is cycled off more than its normal cycling would have had the compressor off, demand relief results. It is this demand relief that utilities desire when they control the load. If the control is too high of a percentage, the home heats up too fast, and residents become uncomfortable and go off the program that enables the demand relief. If the control is at too low of a percentage, no demand reduction is realized. The optimum cycle percentage is dependent on the degree to which a cooling system is oversized to the cooling load.
More advanced cycling techniques have been implemented with TRUECYCLE® and SMARTCYCLE®. These are advanced cycling techniques that monitor the control circuit of the cooling system, and can adjust the operation pattern of the compressor based on forecast runtime. The load control receiver (LCR) knows when the compressor is running and so can over time develop a runtime profile, or over-sizing factor, for the home/compressor combination. TRUECYCLE® and SMARTCYLE® are examples of commercialized approaches for assessing this over-sizing. Once the LCR learns the over-sizing, it uses this information to adjust control times. For example, a 50% TRUECYCLE® on a compressor that would have run for 80% of an hour, because it is oversized for that hour, will be have load shedding of 60% of the hour (the 20% that it was going to be off plus 50% of the 80% it was going to run).
TRUECYCLE® runtime cycle control is a method of increasing the yield of cycled cooling control by adjusting the controlling cycle time on an individual cooling system basis. The control device typically makes this adjustment based on compressor runtime data collected the hour prior to the start of the control period, applying the cycle rate to this runtime instead of clock time. TRUECYCLE® works well for a single speed compressor, where the load is either on or off. But dual speed, dual stage (such as those found on commercial rooftop units), multi-speed, or variable speed compressors can be on at different rates of energy consumption. Thus, knowing just on or off for this type of system is not sufficient to allow one to know the over-sizing. As such, there is a need for a new technique to assess the over-sizing in these types of cooling systems.
Additionally, a problem unique to expanding load shedding arrangements to small energy consumers is the problem of accounting for the savings gained by the load shedding and accurately crediting these savings to the end consumer. Previously disclosed systems for controlling power consumption by small energy consumers approach this problem by ensuring the loads are kept below a pre-determined or broadcasted level. In U.S. Pat. No. 6,806,446, Neale teaches a system that controls the overall power consumption of a building by measuring and recording the power drawn by various appliances, including the air conditioner, and using this historical data to decide whether appliances can be turned on based on the available power. In U.S. Pat. No. 6,975,926, Schanin teaches a system for using the historical performance of a cooling system to control its performance such that the cooling output is held within an acceptable temperature range while providing a reduction in power consumption during the controlled time period.
These previously disclosed systems and methods have several common characteristics. They use historical performance data to help control cooling systems such that power consumption limitations may be observed during specified periods of time. None allow for the utility company to actively monitor compliance with broadcasted load shedding requirements, nor do they allow for any accounting of such compliance. Further, with previously disclosed systems, a utility is unable to differentiate between those consumers who have the ability to provide more shedding than others. An effective solution to these problems that would permit utilities to take advantage of the large potential for load shedding represented by the small energy consumers in such a way as to equitably account for the imposition of the load shedding obligations would be desirable.